Wood-boring and wood-feeding insects, most notably termites, damage and destroy real property and natural resources in the United States every year. Although the extent of the harm shows regional disparity, the annual, direct cost attributable to these creatures across the nation is measured in the billions of dollars. Indeed, the United States Department of Agriculture estimates that the damage caused by the Formosa Subterranean Termite alone exceeds one billion dollars annually.
If the wood-destroying insects are detected, various extermination methods are demonstrably effective at curing the infestation problem which prevents further damage. Chemical and thermal treatments of infested wooden objects, for example, produce positive results as extermination techniques. However, timely insect detection itself has emerged as the better part of the challenge to mitigating termite damage.
The most common method in use today to detect termites is visual inspection. Observables such as surface ridges corresponding to internal termite tunnels, or termite waste (sawdust) may be evident at or near the infested areas. In extreme cases, the integrity of a wooden object may be compromised, or structural failure has already occurred.
As conclusive as visual evidence may be, the problems with the visual inspection technique are manifold. First, the test's subjective nature promotes inconsistent results that are neither reproducible nor lend themselves to error analysis. Second, and particularly important in the context of habitable structures, situations may exist where the infested area is not readily accessible, or where visual inspection is otherwise hampered by wallboard, plaster, insulation and the like. And third, by the time the infestation indicators are prominent enough to warrant attention, the lion's share of the damage may have occurred.
One line of inquiry has focused on the sonic signatures generated by insects in infested sites. This method monitors the human-audible portion of the spectrum (20 Hz to 20,000 Hz) to detect insects feeding on an agricultural sample of interest. However, this method suffers from a low signal-to-noise ratio since the audible portion of the spectrum necessarily records high levels of background unrelated to the insects' activities.
Another line of inquiry has been Acoustic Emission. “Acoustic Emission” (AE) may be defined as the release of elastic energy by a material as that material undergoes deformation. The deformation induces In-Plane (IP) and Out-Of-Plane (OOP) waves by causing mechanical disturbances within the material at points of inhomogeneity. For example, AE is widely employed as a non-destructive technique to detect cracks in metal parts and plate-like metal structures. To test the material, a static stress is applied to the part or plate under scrutiny. The applied stress causes the cracks, if any, to be fractionally extended. The extension of the cracks liberates energy at the leading edges of the cracks, and the energy propagates through the material as mechanical waves.
Advances in AE technologies, particularly the construction of transducers having peak sensitivity at ultrasonic frequencies, facilitated the development of the first-generation, AE-based termite detector.
These devices converted the OOP mechanical wave caused by termites tearing the wood during feeding into electrical signals later manipulated electronically. However, the accuracy of all devices based on AE have been unsatisfactory and, for the reasons described below, AE based devices suffer from most of the same drawbacks as their sonic progenitors, as well as the inadequacies of the visual inspection method. First, the devices were only capable of detecting the stress waves produced by feeding termites. Stress waves caused by termites feeding are the highest amplitude stress waves produced by an active colony; thus greater sensitivity is required to detect other forms of termite activity such as their movement within an infested object.
Second, the sensitivity of the first generation termite detectors is low. Independent studies testing the sensitivity of the instruments found that the devices, placed 50 cm from a feeding termite colony, only detected the activity about half the time. Furthermore, even in those tests where the detector was placed in close proximity to the activity, the recorded count rate was only between five and twenty five counts per minute. Therefore, background events remain a concern even though the background is reduced in the ultrasonic frequency range.
Third, the first-generation termite detectors are sensitive to the (OOP) waves alone. The transducer is placed in direct contact with the surface of the wood and will detect only the OOP waves, traveling across the grain of the wood rather than the IP waves traveling along the grain of the wood. As a consequence of preferentially detecting the OOP waves, the signal strength (and by corollary the instrument's sensitivity) is less than optimal because the attenuation of stress waves propagating across the grain of the wood to produce an (OOP) displacement at the surface of the wood, is much greater than the attenuation for stress waves propagating along the grain of the wood (IP).
Finally, because the devices require the transducer to be in contact with the surface of the material under scrutiny, the first-generation instruments are limited in a manner analogous to the limitations of the visual inspection technique. Specifically, tests on most areas within habitable structures are hampered by difficulties of access, or by barriers to surface contact such as drywall and the like.